Inside the JavaScript parser, a cast of an integer to a narrower type can result in data read from outside the buffer being parsed. This usually results in a non-exploitable crash, but can leak a limited amount of information from memory if it matches JavaScript identifier syntax. This vulnerability affects Firefox < 56.

References

• https://nvd.nist.gov/vuln/detail/CVE-2017-7813
• https://bugzilla.mozilla.org/show_bug.cgi?id=1383951
• https://www.mozilla.org/security/advisories/mfsa2017-21/
• http://www.securityfocus.com/bid/101057
• http://www.securitytracker.com/id/1039465